Allow filtering results by items in specified organization (see #23)

Uses lobid-organisations auto-suggest API to select the organisation
Support multiple item owners, display owner names, tweak UI (#23)
Store item owners filter in session, fix filtered type facets (#23)
Tweak facet filter for multiple owners, benchmark faceting (#23)
Asynchronous facet loading to avoid UI blocking (#23)

In particular with multiple owner filters facets can take a while,
so we load them asynchronously with an ajax call to a facets route
Display type label in faceted search, not URI (#23)
Make tag icon transparent (#27)